THE EFFECT OF PLANE OF NUTRITION AND SLAUGHTER WEIGHT ON GROWTH AND FOOD EFFICIENCY IN BULLS, STEERS AND HEIFERS OF 3 BREED CROSSES

The effects of plane of nutrition and slaughter weight on the growth r ate and food efficiency of cattle with a range of growth potentials an d which had been reared on high-forage diets were examined using 236 F riesian, Limousin X Friesian and Belgian Blue X Friesian bulls, steers and heifers. They were reared under uniform management until 12 to 13 months of age and were then given a complete diet consisting of grass silage and concentrates (2:1 on DM basis) either ad libitum or at 80% of ad libitum intake. Bulls, steers and heifers were initially 394, 3 62 and 360 kg live weight. Representative groups were slaughtered init ially and serially, bulls at 560, 610 and 660 kg live weight, steers a t 510, 560 and 610 kg and heifers at 460, 510 and 560 kg. Reducing foo d intake reduced live-weight, carcass, lean and fat gains by 28, 27, 2 2 and 37%, respectively, the effect on lean gain being significantly g reater in bulls than in steers or heifers. Responses in lean gain to i ncreasing food intake were 5.1, 3.1 and 2.1 g/MJ metabolizable energy for bulls, steers and heifers, respectively. It is concluded that for animals reared on high-forage diets which sustained moderate growth ra tes during the growth phase and which were slaughtered at moderate lev els of fatness, restricting dry matter intake during the finishing sta ge tended to reduce the efficiency of carcass gain and did not affect the efficiency of lean gain. There were significant interactions betwe en plane of nutrition and the sex of the animals for rate and efficien cy of lean gain, but there were no significant interactions between pl ane of nutrition and either slaughter weight or breed.